1. Field of the Invention
The present invention relates to improved processes for the production of polychlorotrifluoroethylene homopolymers and copolymers; more particularly the present invention provides a novel redox system for the production of polychlorotrifluoroethylene homopolymers and copolymers which may be ultimately used in the formation of formed articles, i.e. films, resins, thermoformed articles, and the like wherein the polychlorotrifluoroethylene homopolymers and copolymers produced according to the present invention feature among other attributes, improved processability. The improved production process further provides an aqueous shelf stable suspension of polychlorotrifluoroethylene homopolymers and copolymers.
2. Description of the Prior Art
The preparation of solid polymers of polychlorotrifluoroethylene (hereinafter sometimes referred to as "PCTFE") is well known to the art as well as are PCTFE materials which further comprise copolymers including but not limited to CTFE-vinylidene fluoride, CTFE-tetrafluoroethylene, as well as CTFE-ethylene copolymers. These materials are described in detail, for example, in the Encyclopedia of Polymer Science and Engineering, Sec. Ed. Vol. 3, at Page 463, Pub. John Wiley and Sons. As therein described, articles and films formed from PCTFE materials exhibit desirable vapor barrier properties, good thermal stability and resistance to strong oxidizing agents.
There are presently known a plurality of processes which were suitable for the formation of the homopolymer, polychlorotrifluoroethylene. High molecular weight homopolymers and copolymers of PCTFE may be prepared by free radical initiated polymerization either as bulk, suspension, or aqueous emulsion via the use of a suitable initiator system or in the alternative by ionizing radiation.
For the formation of PCTFE in an aqueous suspension process, a redox initiator system which comprises, for example, an alkaline metal persulfate as an oxidant, an alkaline metal bisulfite as a reductant, and metal salts such as ferrous sulfate, silver nitrate, or copper sulfate, which are known to be useful as accelerators to the redox reaction, may be used.
For the production of PCTFE by emulsion polymerization, emulsifiers, generally fluorocarbon and chlorofluorocarbon compatible emulsifiers, may be used.
For the formation of PCTFE by bulk polymerization, a peroxide may be used as an initiator. Of particular note, useful peroxides which may find use are one or more of the group of: trichlor-acetal-peroxide, dichlorotrifluoro-propionyl peroxide, heptafluorobutyryl peroxide, as well as other acyl peroxides derived from fluorocarboxylic acids.
Also known to the art is the formation of copolymers of PCTFE, such as copolymerized PCTFE with vinylidene fluoride and/or tetrafluoroethylene which may be produced by either suspension or emulsion polymerization processes. Generally, however, the use of comonomers is such that the weight percentage of the comonomers is relatively low, i.e., generally comprising a minor proportion of the total polymer as it is known that the desirable vapor barrier properties are substantially degraded by the inclusion of excessive amounts of comonomers.
Other methods for the production of PCTFE homopolymers and copolymers (which are sometimes hereinafter generally referred to as "PCTFE polymers") include the processes for the production of PCTFE film forming and copolymer resins as described in U.S. Pat. Nos. 2,705,706; 2,689,241; 2,569,524; 2,783,219; 2,820,026; 3,640,985; 3,671,510; 3,642,754; 3,632,847; and 3,014,015.
While these processes provide useful methods for the production of PCTFE polymer resins, various shortcomings in one or more of these processes have compelled the development of further novel methods for the production of PCTFE polymer resins.
One such shortcoming in the prior art is the encapsulation of inorganic moieties from the initiating species which are known to produce a PCTFE polymer resin with a relatively high content of residual ash, which limits the range of applications within which articles formed using the PCTFE polymer resin may be used. Such materials are known to have lower dielectric strengths which is often undesirable for use in electrical and electronic devices and/or packaging.
A further shortcoming in the prior art is that processes which may be used to form suspensions of PCTFE polymer resins typically require the use of a soap or a surfactant composition.
Therefore, it will become apparent to those skilled in the art that there remains a present and continuing need for the provision of improved PCTFE homopolymers and copolymers which are suitable for the production of formed articles therefrom, or to be included in the structure of a formed article. There also remains a continuing need in the art for the production of improved PCTFE homopolymers and PCTFE copolymers which feature improved machine processability, particularly in regard to conventional thermoforming and process equipment.